Surface structure and composition play a major role in defining many of the physical properties and ultimate uses of solid organic polymers. In engineering and biomedical applications, features such as wetting, weathering, adhesion, friction, electrostatic charging, permeation, biocompatibility, and bacterial fouling are largely dependent on surface properties.
Surface properties determine the value in use of many solid materials. These properties are as critical as such bulk parameters as strength, elongation, Young's modulus, etc. Furthermore, full utilization of bulk properties is often supported by surface characteristics which are mainly dependent on surface energetics (e.g., surface tension). This is particularly the case for fibers, films, membranes or other forms where the surface area/volume ratio is large.
There is much interest in modifying the surface of common polymers to make them more hydrophilic. Examples of benefits of such modification include wettable surfaces for dye adsorption, better adhesion to metal films, enzyme immobilization, weathering, blood compatibility and less tissue damage for intraocular lenses. However, most methods used involve creating a thin oxidized layer of the base polymer by plasma or flame treatment or by generating polymers near the surface which may be only physically adsorbed, e.g., radiation grafting. Both methods can cause damage to the bulk properties (e.g., tensile strength) and yield a variable and poorly characterized surface attachment.
Low density polyethylene (LDPE) is the largest volume plastic produced in the world. However, it has a slick, oily surface upon production which is hydrophobic. For many of the reasons given above, it would be highly advantageous to form a hydrophilic surface on LDPE without affecting any of the bulk properties. The process of this invention accomplishes such a goal.
The invention comprises a process whereby LDPE in various forms (film, fiber, granule, powder) is treated to produce a surface that is highly hydrophilic. The process results in a LDPE substrate with acrylamide grafted onto the surface by covalent bonds. The process involves first oxidizing the surface of LDPE, then reducing to form an hydroxyl-rich surface, and finally graft polymerizing acrylamide to the surface through free-radical initiation. The oxidation step is performed by exposure to chromic acid, the reduction step by exposure to diborane. The step of graft polymerization is performed using ceric ammonium nitrate as a free-radical initiator.
It is known in the art to oxidize LDPE by chromic acid. D. Dwight, in Chemtech, p. 166, March, 1982 and J. R. Rasmussen, et al., in J. Amer. Chem. Soc., 99, 4736 and 4746 (1977), review the technique and provide optimum conditions to maximize carbonyl production and reduce bulk damage. It is also known to use free-radical initiation, including the use of ceric ammonium nitrate, to graft polymerize vinyl monomers onto hydroxyl-bearing surfaces. See, for example, the article by G. Mino et al., J. Polym. Sci., 122, 242 (1958). Prior art, however, teaches the use of this technique on substrates that are swollen with solvent, thereby affecting the bulk properties of the substrate. The grafting performed by the process of the present invention occurs only on the surface, as the substrate is non-swollen during the grafting step.
U.S. Pat. No. 4,080,405 to Agouri et al. discloses a process for chemical modification of polyolefins by grafting polar monomers to the polyolefin in an aqueous dispersion by means of a free-radical generator. Acrylamide is taught as a possible monomer and polyethylene is taught as a possible substrate. However, the teachings of Agouri et al. differ from the present invention in that Agouri et al. teaches solution polymerization. In solution polymerization, the polymer (e.g., polyacrylamide) forms in the solution and is then grafted by physical adsorption onto the surface of the substrate. Solution polymerization results in a much larger weight gain than the method of the invention and much weaker bonding. The method of Agouri et al. also requires additional additives and is performed entirely at elevated temperatures, with a swollen substrater, thus affecting the bulk properties of the substrate. This invention calls for performance of the graft polymerization step at room temperature.
Other U.S. Patents of possible interest are U.S. Pat. Nos. 3,796,773 to Coleman, 3,652,730 to Favie et al. and 3,644,581 to Knaack.
It is the general object of this invention to produce LDPE having a hydrophilic surface.
It is a further object of this invention to produce LDPE having a hydrophilic surface by a method which affects only the surface properties of the LDPE.
It is a further object of this invention to produce LDPE having a hydrophilic surface more stable and durable than that produced by alternate methods.
It is a further object of this invention to produce LDPE with acrylamide grafted onto the surface by covalent bonding to form a stable, durable hydrophilic surface.